Magnetic Random Access Memory (MRAM) is a non-volatile memory technology that is gaining popularity in the computer market. Unlike other memory technologies (e.g., SRAM, DRAM, FLASH, etc.) that store data as electric charge or current flows, MRAM stores data as a magnetic state in magnetoresistive storage elements. Typically, a magnetic bit includes two ferromagnetic layers (or structures), each of which can hold a magnetic field that has one of two possible polarities. One popular example of magnetic bit is a magnetic tunnel junction (MTJ), which includes a free magnetic layer for data storage and a pinned magnetic layer for reference, separated by a thin insulating barrier layer through which a tunneling current may flow. The logic state of MTJ depends on a relative polarity of the free and pinned magnetic layers. For example, if the free and pinned magnetic layers have the same polarity, the MTJ may be storing a logic state “0.” As another example, if the free and pinned magnetic layers have an opposite polarity, the MTJ may be storing a logic state “1.”
The MRAM may determine (“read”) the logic state of a given magnetic bit by passing a read current through the given magnetic bit and then determining a resistance of the given magnetic bit, which indicates the relative polarity of the free and pinned magnetic layers (e.g., a lower resistance typically indicates the same relative polarity and a higher resistance typically indicates an opposite relative polarity). In one example, the MRAM may pass the read current through the given magnetic bit by sending the current through a conductor (e.g., a bit line) coupled to one side of the given magnetic bit and switching on a selection transistor coupled to the other side of the given magnetic bit, and the MRAM may determine the resistance of the given magnetic bit by measuring the resulting current and/or voltage. Other examples for reading the given magnetic bit may exist as well.
The MRAM may store (“write”) data to a given magnetic bit using a few different techniques. According to one technique, the MRAM may write data to the given magnetic bit by applying magnetic fields that couple to the given magnetic bit's free magnetic layer. The MRAM may generate these magnetic fields via write currents running through conductors arranged above and below the given magnetic bit. In one example, an MRAM may include a first write line arranged above the given magnetic bit and oriented in a first direction and a second write line arranged below the given magnetic bit and oriented in a second direction that is perpendicular to the first direction.
According to another technique known as spin-torque transfer (STT), the MRAM may write data to the given magnetic bit by passing a spin-polarized current through the given magnetic bit that is capable of changing the polarity of the given magnetic bit's free layer. In this respect, if the spin-polarized current electrons have to change their spin upon entering the given magnetic bit, those electrons may generate a torque that changes the polarity of the given magnetic bit's free layer. In one example, the MRAM may pass the spin-polarized current through the given magnetic bit by sending the current through a conductor (e.g., a bit line) coupled to one side of the given magnetic bit and switching on a selection transistor coupled to the other side of the given magnetic bit. Typically, an MRAM employing STT uses the same conductor and selection transistor for reading and writing the given magnetic bit.
An MRAM with STT magnetic bits may demonstrate various benefits over other MRAMs, such as higher magnetoresistance, higher signal levels, and lower write currents. However, an improved structure for reading and writing STT magnetic bits is desirable.